1. Field of the Invention
The present invention relates to a liquid crystal display device and a manufacturing method thereof.
2. Description of the Related Art
Conventionally, as a liquid crystal display device used in a television receiver set or the like, there has been known a liquid crystal display device where a switching element such as a thin film transistor is provided for each pixel, and driving of a desired pixel is controlled by a switching operation of the switching element. Such a liquid crystal display device includes a liquid crystal display panel where a liquid crystal material is held between a TFT substrate on which thin film transistors are formed and a counter substrate on which color filters are formed.
With respect to the TFT substrate and the counter substrate, usually, 6 to 8 sheets of photo masks are used for manufacturing the TFT substrate. Exposure treatment using these photo masks and development, film peeling and etching treatment relating to the exposure treatment are factors which increase a manufacturing cost. That is, when a large number of photo masks are used, a manufacturing cost of a liquid crystal display device is pushed up. Accordingly, to reduce the manufacturing cost, there has been a demand for a liquid crystal display device and a manufacturing method thereof which can reduce the number of photo masks used in the manufacture of the liquid crystal display device to 5 or 4 sheets.
With respect to a liquid crystal display device, there has been known a liquid crystal display device having the constitution where source electrodes, pixel electrodes and semiconductor layers are present on the same layer, and the source electrodes and the pixel electrodes are directly connected with each other in a conductive manner not via contact holes.
In the liquid crystal display device having such a constitution, among defects which occur in the manufacturing steps, the number of disconnection defects of video signal lines is large. That is, the liquid crystal display device has a drawback that the disconnection defects of the video signal lines lower a yield in the manufacture of the liquid crystal display device.
As a cause of such disconnection defects of the video signal lines, considered is a case where when the pixel electrodes are formed after forming the video signal lines, due to the impregnation of an etchant used at the time of forming the pixel electrodes, the video signal lines are dissolved. To overcome the above-mentioned drawback, there has been proposed a liquid crystal display device and a manufacturing method thereof which can reduce the number of photo masks and also can reduce disconnection defects of video signal lines (see JP 2005-157016 A, for example).
In the liquid crystal display device and the manufacturing method thereof described in JP 2005-157016 A, the number of photo masks is reduced by performing collective etching treatment which uses a half exposure technique, and the number of disconnection defects of video signal lines is reduced by forming video signal lines after forming pixel electrodes.
However, in the liquid crystal display device and the manufacturing method thereof described in JP 2005-157016 A, scanning signal lines are formed after forming semiconductor layers and hence, the semiconductor layer is formed with a width equal to or slightly narrower than a width of a gate electrode whereby the requirement on accuracy of mask alignment at the time of forming a source-drain line formed on the semiconductor layer becomes strict thus causing a drawback that a process design becomes difficult eventually.
It may be possible to cope with such a drawback by reducing a film thickness of a resist pattern using oxygen plasma treatment with strengthened anisotropy or by making a design in which a size change amount of the resist pattern is taken into account. However, the difficulty in process design remains even in such a case.